Posttraumatic Stress Disorder (PTSD), a highly prevalent, chronic psychiatric disorder, often co-occurs with other psychiatric disorders, is associated with occupational and social dysfunction, and can lead to chronic disability. PTSD is also a risk factor for a number of health-related concerns including chronic pain and cardiovascular disease and is associated with significant economic burden. Veterans are at increased risk for PTSD with prevalence rates up to 3 times those who are non-Veterans. Existing PTSD treatments have been found to be less effective for Veterans with PTSD, in fact, combat-related trauma has been associated with the lowest treatment effect sizes in studies of PTSD treatments. Veterans with PTSD often experience difficulty engaging in traditional CBT exposure based approaches, which can trigger the uncomfortable emotional arousal they want to avoid. Further, the repeated nature of trauma exposure resulting from multiple and longer deployments, mental health stigma within military culture, and administration time, resources, and costs for mass populations such as the military, all contribute to decreased implementation and possibly efficacy of existing PTSD treatments. Therefore, novel and adjunctive interventions for the treatment of PTSD are sorely needed for Veterans with PTSD. The aim of the proposed research is to examine the potential of exercise as an adjunctive intervention for PTSD in Veterans. There is a growing body of literature that supports the efficacy of exercise interventions in treating clinically diagnosed populations with various psychiatric disorders, and increasing physical activity has the potential to be cost-effective, flexible, and accessible. Veterans may find engaging in exercise as a more acceptable, less stigmatizing approach to managing PTSD symptoms. We propose to conduct a small, randomized clinical trial with 50 Veterans in treatment for PTSD randomized to receive either an adjunctive aerobic exercise (AE) intervention or a contact control health education comparison (HEC) intervention. The pilot RCT will allow us to 1) determine the feasibility of recruitment and randomization, 2) determine the acceptability of the AE and HEC conditions, and 3) to provide preliminary evidence of the effects of AE. This project will thus lay the groundwork for a subsequent randomized controlled trial with adequate statistical power to test the efficacy of AE in reducing PTSD symptoms, decreasing anxiety and depressive symptoms, and increasing quality of life and cardiorespiratory fitness in Veterans with PTSD. The longer term significance of this project lies in the potential for adapting the AE intervention for use in VA hospitals nationwide. The larger aim of this program of research is to disseminate an effective, aerobic exercise intervention that can be readily integrated into PTSD treatment programs across treatment settings within Veterans Administration (VA) hospitals. Future dissemination of this approach could result in high public health significance in decreasing the overall severity of PTSD symptomatology in Veterans, thereby reducing PTSD-related morbidity.